Subterranean drilling systems that employ downhole drilling motors are commonly used for drilling boreholes in the earth for oil and gas exploration. FIG. 1 shows an isometric view of a prior art subterranean drilling system 10. The subterranean drilling system 10 includes a housing 18 enclosing a downhole drilling motor 12 (i.e., a motor, turbine, or any other device capable of rotating) that is operably connected to an output shaft 14 and a thrust-bearing apparatus 16 assembled to the downhole drilling motor 12. A rotary drill bit 20 configured to engage a subterranean formation and drill a borehole is connected to the output shaft 14. The rotary drill bit 20 is shown as a “roller cone” type bit including a plurality of roller cones 22. However, other types of rotary drill bits, such as so called “fixed cutter” drill bits are also commonly used. As the borehole is drilled, pipe sections may be connected to the subterranean drilling system 10 to form a drill string capable of progressively drilling the borehole to a greater depth within the earth.
The thrust-bearing apparatus 16 includes a stator 24 that does not rotate and a rotor 26 that is attached to the output shaft 14 and rotates with the output shaft 14. The stator 24 and rotor 26 each include a plurality of bearing elements 25 that may be fabricated from superhard (polycrystalline diamond) compacts for providing diamond bearing surfaces that bear against each other during use.
In operation, high pressure drilling fluid is circulated through the drill string and power section (not shown) of the downhole drilling motor 12, usually prior to the rotary drill bit 20 engaging the bottom of the borehole, to generate torque and rotate the output shaft 14 and the rotary drill bit 20 attached to the output shaft 14. The drilling fluid is also used for lubricating and cooling the bearing surfaces of the bearing elements 25 during operation. The housing 18 of the downhole drilling motor 12 remains stationary as the output shaft 14 rotates the rotary drill bit 20. When the rotary drill bit 20 engages the bottom of the borehole, a thrust load is generated that tends to push the rotary drill bit 20 and the drill string downhole to generate what is commonly referred to as “on-bottom thrust.” The on-bottom thrust is carried, at least in part, by the thrust-bearing apparatus 16. In the absence of on-bottom thrust, fluid flow through the power section may cause what is commonly referred to as “off-bottom thrust.” The drilling fluid used to generate the torque for rotating the rotary drill bit 20 exits openings formed in the rotary drill bit 20 and returns to the surface, carrying the cuttings of the subterranean formation through an annular space between the drilled borehole and the subterranean drilling system 10.
Both the off-bottom and on-bottom thrust carried by the thrust-bearing apparatus 16 can be extremely large. Accordingly, the operational lifetime of the thrust-bearing apparatus 16 often determines the useful life for the subterranean drilling system 10. During operation, bending and/or side loading of the downhole drilling motor 12 can cause some of the bearing elements 25 to experience higher than desired stresses and temperatures. A slight angular misalignment between the stator 24 and rotor 26 of the thrust-bearing apparatus 16 due to such bending and/or side loading may cause certain bearing elements 25 to partially contact or even fail to contact each other during operation, causing other bearing elements 25 to contact each other at higher than desired stresses. The bearing elements 25 that remain in contact with each other may experience sufficiently large stresses and sufficiently high temperatures during drilling to damage or even fracture the bearing elements 25 and, ultimately, may cause premature failure of the downhole drilling motor 12.
Therefore, manufacturers and users of subterranean drilling systems continue to seek improved thrust-bearing apparatuses that can accommodate angular misalignment between stator and rotor components.